If the big backup is still running, you could look at the live log About → Show logs → Live → Verbose
2019-05-29 11:00:27 -04 - [Verbose-Duplicati.Library.Main.Operation.Backup.FilePreFilterProcess.FileEntry-CheckFileForChanges]: Checking file for changes C:\PortableApps\Notepad++Portable\App\Notepad++\backup\webpages.txt@2019-05-26_065040, new: False, timestamp changed: True, size changed: True, metadatachanged: True, 5/29/2019 2:59:43 PM vs 5/29/2019 12:32:57 PM
is an example of how Duplicati looks at a variety of factors to see if it should examine a file for changes, however if it’s actually uploading file data then it must be finding big changes as it reads through files…